Photocatalytic water splitting has been studied as a way to harness solar energy by using it to generate clean, high energy-containing hydrogen from water, an abundant, inexpensive feedstock. Efforts have been directed toward producing compounds with higher catalytic activity in the photolysis of water. Catalytic activity of the titanium dioxide-based photocatalysts originally studied was improved with catalysts such as Pt/TiO2 and RuO2/TiO2. Strontium-titanium oxide-based materials such as a reduced SrTiO3/platinum electrode pair, SrTiO3 powder modified with rhodium oxide, platinized SrTiO3, and nickel-loaded SrTiO3 have been studied, but the amount of absorbed photons used in the photolysis for these photocatalysts (the “quantum yield”) is less than 1%. More recently, quantum yields of 5-10% have been obtained with layered structures of K4Nb6O17, K4TaxNb6-xO17, and Rb4TaxNb6-xO17, and quantum yields as high as 30% have been obtained with K2La2Ti3O10 prepared in a polymerized complex method. The materials with improved quantum yield have interlayer reaction sites that can physically separate electron and hole pairs created by photoabsorption to retard electron-hole recombination. Even higher photocatalytic activity of the complex oxides would be desirable, however.
Complex oxide photocatalysts for water splitting have been prepared by several methods. In one method, the photocatalysts are prepared by grinding powder precursors, followed by calcination at high temperature. Conventional ball mill grinding requires a long processing time, however, after which the average particle size of the powder is still several microns. High-density yttrium-stabilized zirconia (YSZ) balls have been used for grinding. Another preparation method, the sol-gel process, is complicated and easily contaminated. Further, conventional grinding methods require severe calcinations conditions to optimize catalyst activity. Moreover, the catalysts prepared have suffered from poor homogeneity and poor crystallinity.
Thus, a need remains for a straightforward process for making photocatalytically active materials, as well as for such materials having higher catalytic activity.